Fluids in the oil and gas industry, such as crude oil or natural gas liquids (NGLs), are shipped under high pressure through lengths of pipe in remote areas. These fluids, also known as dirty hydrocarbons, are regularly analyzed by on-line analyzers as they pass through the pipes. The results are used to optimize blending stations and to ensure tariff limits are not exceeded at receipt points and at major rail car terminals.
A sample of fluid from a high pressure pipe needs to be conditioned before being analyzed by an on-line analyzer. Sample conditioning systems present some challenging design requirements. The process or pipe carrying the fluid of interest can vary widely in terms of pressure, temperature, composition, quality and contamination of the fluid. However, the on-line analyzing device is making sensitive measurements of the variable of interest, and requires that all other variables be tightly controlled. Sample conditioning typically requires pressure reduction, temperature alteration, flow control and implementation of safety mechanisms to ensure the measurement device is not damaged and personnel are not injured.
Conventional on-line analyzer sampling systems are designed for a continuous flow of fluids through filters and the analyzer. These include analyzers for pH, oxygen reduction potential, and dissolved oxygen. However, newer analyzing devices, such as Gas Chromatographs and Vapor Pressure Analyzers, are cyclic analyzers that only grab samples for very short periods of time and require that no fresh sample be taken for comparably long periods of time.
As mentioned above, sampling systems require sample conditioning before measurement, such as cooling, heating, and/or dilution. Because of these sample modifications, the benefits of cycling analyzers can outweigh any drawbacks of discretely sampling of fluids, rather than using continuous or live measurement. Further, in some processes, speed of response may be considered a lower priority than reliability and quality of the results.
Further, when an on-line analyzer fails, is inoperable, or requires maintenance, the pipeline system experiences downtime, which costs the user time and money. Daily maintenance is also exceedingly difficult because of the remote location of these on-line analyzers.
The majority of the system downtime is related to the sample conditioning portion of the on-line analyzer. It is generally known that 80% of all analyzer failures can be attributed to the sample conditioning system, and when dealing with dirty hydrocarbons this percentage can be even higher.
Because the pipe pressure is in significant excess of the on-line analyzer's pressure capabilities in many applications, the first element of conditioning the sample is typically pressure reduction. This can be achieved through the installation of an adequately specified pressure regulator to solve the pressure reduction requirement. Unfortunately, restrictive orifices of any kind, like regulators or needle valves, will plug in a short amount of time in crude oil applications immediately downstream from the restriction leading to failure or inoperability of the sample system.
The high rate of plugging at flow restriction points is due to certain components within crude oil, such as asphaltenes and paraffins (wax), which have a high tendency to precipitate where the flow regime is substantially altered. This includes restrictive elements of the sampling system that affect the flow of the sample. Therefore, pressure regulators, flow control valves, orifices, variable area flowmeters and filters all result in precipitation of these components and a build-up of deposits.
As the deposits at flow restriction points increase, they become so persistent that they can plug flowing systems completely, necessitating cleaning by manual, mechanical, or chemical means. Plugging of this nature has been found to occur in traditional systems in as little as a few minutes in extreme cases, but is commonly on the order of 2 hours to 3 days.
If the plug occurs in a filter assembly, it is an inconvenience because it results in downtime and lost revenue, and loss of the benefit from the measuring device. But, clearance of a filter is a relatively straight forward operation for the technician because the filter is typically selected with a design that is easily maintained. However, should the plug occur in the regulator or flow control device, it becomes a much more arduous task to clear as these devices are not designed to be easily serviced or disassembled. Often they require specialty tools, non-reusable gaskets, seals, o-rings and diaphragms that can be easily damaged and are not intended for regular re-building. Threads gall and/or sealing faces may become scratched and hidden flow passages are small and difficult to mechanically clear.
To date, all traditional methods employed for pressure reduction involve using a small orifice, like a needle valve, regulator or capillary to reduce the pressure. These all tend to plug because of the pressure drop or increased turbulence at the orifice, which causes the waxes and asphaltenes to precipitate and, in turn, eventually plug the sample flow.
Known methods of addressing some of these problems include an automatic backflush system to clear filters, but this only addresses the filter assembly and not the pressure regulation system. Some in the industry have suggested operating certain analyzers at a higher temperature to reduce wax build-up and then correlate the result back to the ASTM International method at 40° C. However, high temperature would reduce the wax precipitate build-up on the sensor, but does not address other sample conditioning challenges.
Hydrogen sulfide (H2S) is a lethal gas and human exposure can lead to serious consequences. It can be found in crude oil and other liquid hydrocarbons and is especially dangerous where personnel expect the fluid to be non-hazardous and free of H2S. Many applications require the concentration of sulfur, typically from H2S, to be below 1 or 2 ppm to be considered “sweet” and acceptable for shipping by rail car, truck or to be processed in many facilities. Currently, there is no device that is able to provide reliable measurement of H2S in crude oil and liquid hydrocarbon streams.
Accordingly, there is a need and desire to provide an apparatus and method that can ameliorate or overcome the shortcomings of the prior art.